The study aims to ascertain whether the sole replacement of tenofovir with abacavir once a day improves the immunological response obtained with tenofovir + ddI or whether it is better to perform a double replacement of tenofovir and ddI with abacavir + lamivudine (joint formulation) in a single daily dose to achieve these objectives.

Different works have shown a high rate of virological failure among patients on abacavir + lamivudine + tenofovir or ddI + 3TC + tenofovir, thus rendering the use of these combinations actively unadvisable.

Furthermore, recent studies have also shown that ABC+3TC are associated with a significantly higher increase in CD4 than the current treatment standard formed by AZT+3TC. This provides us with grounds to suppose that patients with TDF+ddI may recover their CD4 with ABC+3HT. Similarly, and recently, the existence of pharmacokinetic interactions between tenofovir + abacavir has begun to be questioned.

Finally, the replacement of tenofovir with abacavir or tenofovir + ddI with abacavir + lamivudine does not detract from the potency of HAART, the toxicity profile is different and their behaviour at mitochondrial level is similar.

This study aims to ascertain whether the sole replacement of tenofovir with abacavir once a day improves the immunological response obtained with tenofovir + ddI or whether it is better to perform a double replacement of tenofovir and ddI with abacavir + lamivudine (joint formulation) in a single daily dose to achieve these objectives.

Inclusion Criteria:

• Age > 18 years.
• HIV-1 infected patients.
• Patients on triple HAART therapy including ddI + tenofovir plus a PI or NNRTI for at least 3 months.
• Patients with an undetectable HIV-1 viral load (< 50 copies RNA / mL or < centre's limit of detection) over the last 6 months.
• Not be on treatment with immunosuppressives, such as: hydroxyurea, interferon, ribavirin or cytostatics.
• Not be on treatment with interleukin-2 or other immunomodulators.
• Women may not be of fertile age (defined as at least one year from menopause or undergoing any surgical sterilisation technique), or must undertake to use a barrier contraceptive method during the study.
• Signature of the informed consent.

Exclusion Criteria:

• Incapacity to give informed consent.
• Bad adherence or treatment interruptions over the previous 6 months.
• Prior exposure to abacavir.
• HAART Therapy including ddI at a dose of 400mg + tenofovir if weight > 60 kg or ddI 250 mg + tenofovir if weight < 60 kg.
• Suspicion of cross resistances to abacavir and lamivudine.
• Hepatic or pancreatic analytical alterations 4 times above the limit of normality.
• Presence of opportunistic infections and/or recent tumours (< 6 months).
• Patients participating in another clinical trial.